Carrier telegraph system



Jan- 26, 1954 l.. A. GARDNER ET AL CARRIER TELEGRAPH SYSTEM A3 Sheets-Sheet l Filed Aug. 7, 1951 m. ...vhx

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LA. GARDNER JL HYS/(0 /NVE/VTORS:

Jan. 26, 1954 L. A. GARDNER ET AL CARRIER TELEGRAPH SYSTEM 3 Sheets-Sheet 2 Filed Aug. '7, 1951 MEMO/5S.. .A. GARDNER J.L. HKS/(40 Jan. 26, 1954 A. GARDNER ET AL CARRIER TELEGRAPH SYSTEM Filed Aug. 7, 1951 5 Sheets-Sheet 3 J. L. HYS/(0 www@ A7' ORNE E MWI.

/NVENTORS Patented Jan. 26, 1954 CARRIER TELEGRAPH SYSTEM Leland A. Gardner and John L. Hysko, Summit, N. J., assignors to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application August 7, 1951, Serial No. 240,628

oiaims. l

This invention relates to carrier telegraph systems and more particularly to frequency shift carrier telegraph systems utilizing separate frequencies for the marking and spacing signals.

An object of the invention is to make use of the inherent characteristics of a frequency shift carrier telegraph system to provide economical supervisory signaling arrangement by building into the carrier terminal means for activating such arrangements.

Other objects of the invention are to automatically shift the carrier frequency of the aforementioned telegraph system by vacuum tube and varistor keying under the control of telegraph direct current signals; to limit the received carrier signals to constant Voltage, and to eliminate bias distortion introduced by noise and extraneous signals.

A feature of the invention is a modulator for switching the carrier oscillator on and off in response to direct-current telegraph supervisory signals.

Another feature of the invention is a varistor network for shifting the carrier oscillator from the marking to the spacing frequency or vice versa under the control of direct-current telegraph supervisory signals.

Another feature of the invention is a break which insures a rapid clean break when the receiving operator desires to stop transmission from the distant terminal.

Another feature of the invention is the floating output circuit which permits the cathodes of the output loop tubes in the receiving circuit to be connected to any direct current potential as required by the loop terminations used.

Another feature of the invention is the unbalanced detectcr circuit which centers the demodulated signals at a voltage negative with respect to the cathodes of the output loop tubes of the receiving circuit yand' thus avoids unbiased signals.

Another feature of the invention is the markhold feature which causes current to flow in the loop circuit when no incoming carrier from the distant terminal is being received.

In the system to be disclosed in this application, carrier telegraph communication between stations is accomplished by shifting the resonant frequencies v1 and v2 of a carrier oscillator (v1 and 112 corresponding to the marking and spacing frequency respectively) by means of a modulator and varistor network responsive to directcurrent telegraph signals. In accordance with the present invention, supervisory` signals are provided between terminals by keying the oscillator to provide on and oli carrier signals, which are transmitted and separately detected in a receiver for detecting the carrier frequencies 111 and v2 at a distant station.

Reception requires means sensitive to frequency changes to distinguish between the spacing and. marking elements of the signals. This is provided in the receiver by a discriminator circuit which consists essentially of two filters, the one responsive to the marking frequency and the other to the spacing frequency. Each of these discriminator filters is associated with a separate varistor rectifier followed by vacuum tube switches for open and closed circuit operation with reference Ito the direct-current telegraph loops.

Referring to the figures of the drawing:

Fig. 1 shows a block schematic diagram of one terminal of a frequency shift telegraph system in accordance with the invention;

Figs. 2 and 3 show respectively the transmitting and receiving circuits thereof;

Fig, 4 shows a modification of the telegraph carrier oscillator.

Fig. 5 shows the relation of Fig. 2 to Fig. 3.

In conventional amplitude modulation telegraph circuits, signals are formed 'by sending out carrier current over the line for marking and interrupting transmission entirely for spacing. In a frequency shift system, such as disclosed herein, the marking condition as denoted by periods of carrier of a particular frequency v1 and the spacing condition of similar periods of a second frequency v2. During telegraph signal transmission periods, the magnitude of the current applied to the carrier line or medium is thus substantially constant whether marking or spacing, only the frequency changing.

Referring to Fig. 1 which discloses one terminal of a complete telegraph system, sending is performed by means of a modulator tube V2 which shifts the frequency of the carrier oscillator source I, for example, by about to 100 cycles at each signal transition. The desired frequency shift is obtained by altering the resonance constants of the carrier oscillator tuned circuit 2 which determine the frequency at which the oscillator works.

Both marking and spacing frequencies are produced by the single electron tube oscillator l Which consists of triode VI and the associated tuned circuit 2 which determines the oscillation frequency.

The marking and spacing frequencies thus produced are amplified by the amplifier 3 and transmitted through send filter 4 whence they are propagated over transmission line 5 to a distant station, central oiiice or teletypewriter switchboard omce and received by receiving 5apparatus substantially corresponding to blocks 6, 'I and 8 which are respectively a receiving filter and amplifier limiter and discriminator circuit.

At an attended TWX station, thepcwenswitch 9 in the oil position shown in Fig. 1, serves to connect the subset i!) to theftransmissionlea'd TR and the send supervision lead `SS not connected to +130 volts. AThe Ysubset -I -is mechanically biased and operates on current and no-current at a 20-cycle rate.

When a supervisory ringing signal isV received from the distant switchboard, it is passed Y through the receiving channel 6, l, 8 and tube vFrequency shift oscillator .and modulator Referring to Fig. 2, wl 1ich shows the transmitting, receiving and supervisory circuits of the system, the carrier telegraph transmitter comprises an voscillator 2B yhaving a :tuned circuit 2l in its feedback path. .The tuned circuit 2i is `tunable to two discrete frequencies v1 kand vz, termed the normal and shifted frequency respectively. The oscillator vi2@ ,generates-the normal frequency v1, determined by yariable vcapacitor C! and the primary 4coil Ll ,of transformer T, which frequency represents the marking condition. For spacing, the frequency of theoscillator is then shifted to yz by the coupling of additional reactance lia-Cz-into the tuhedcir.- cuit. The switching of the Vadded reactance is under the control of varistors22,;22,1wh0se impedance, high or low is determined .by a bias direct-current voltage on leadjM.

The` oscillator tube 2B, which may be half of a twin triode vacuum tube, furnishes the necessary feedback gain to maintain oscillations. The other half `of the twin triodefnamely tube 2 constitutes a buffer amplifier between the oscillator 2c and the line2 5. The outputfrom the oscillator tube ,20 Vsupplies feedback -to its frequency determining tuned circuit 2| through resistor R22 and winding `li---ll of input trans- `former Tl. Resistor R8 serves to suppress parasitic oscillations.

The oscillating voltage which appears-between winding .6--5 of transformer TI r.is applied between the grid and cathode of vtube 2G causing acurrent of the same frequency Vto circulate through the cathode-plate circuit of ythis tube,

which includes condenser C13 and a group `of resistances RI, R2, R3 in series with the cathode. The drop in the latter -is ,applied to winding 4-5 of transformer TI which furnishes the necessary coupling between the cathode and grid to sustain the oscillations. The grid of vtube 2li is suitably biased by the direct-current component of the cathode current through resistance R3.

For the marking condition, the varistors 22, 22 are non-conducting or biased to provide a very high impedance, thereby rendering the secl .ondary of transformer 'Tl ,ineffective and isolating the additional reactance Ilz-"C2, so that the tuned circuit resonates at the marking freqllEnCy 1 V1:2m/L10.

For the ,spacing condition, the varistors conduct and theoscillator frequency is shifted to the spacing frequency v2 by the coupling of added-reactance L2-C2 in parallel into the tuned primary of transformer Ti The switching of the additional reactance into the tuned circuit 2l is accomplished by biasing varistor '22, 22' toa low impedance value by means of the appropriate direct-current bias. If .the net added reactance is capacitive thefrequency is shifted to a lower value. if inductive, the Vfrequency is shifted to a higher value.

The varistors 22, 22' function as series switches under the control of modulator tube 213, which may be half of a twin triode vacuum tube'vfz. As explained later, the gridcf tube ,Z3 is `.driven by the signals originating in the connec ng Ydirect-current telegraph Acircuit 2d. Modulator tube 23 is thus Ycaused to conduct for an outgoing mark and to be 4cutoff for an .outgoing space.

When modulator ltube 23 `is conducting, resistor RIE in series with the plate-cathode circuit of the tube is shunted across resistors .Ri and RH. This condition produces a ,potential at the plate of tube 23 .which is negative with respect to the voltage at the junction of vresistors Rl@ and Ril. This negative .voltage `.is connected between terminal 2 of transformer Tl and terminal .2 of coil L2 and is thus impressed across .varistorsz and 22' in the non-conducting direction.

vFor the spacing condition, the tube 23 iscut off, a current iiows through resistor Riti, into center .tap terminal 2 of input transformer Ti, through yvaristors 2-2 and 2'2 .and through retard coil L2 to the junction of resistors Rilland-Ri l. This current divides and Ahows through each of the nvaristors .22 and 22', providing a positive lbias voltage for the varistors whichmaintains them in a `low impedance condition. Thereby the secondary of input transformer Tl isfeffectively coupled to the reactance of coil ylli. and

capacitor VC2. This results in the oscillator operating at the spacing frequency-r2.

Since the -voltage and current .for controlling the impedance of the varistor units isfed into center-taps,` 2, 2 of vthe input transformer secondary and of the retard coil, respectivelyno ,disturbing transients are coupled into the oscillator tuned circuit.

The marking frequency v1 and -the spacing frequency :1 2 from the oscillator are passed .through capacitor -(gli-Il into butler amplifier 2B', Vthence through send filter .26 and over the trans- -rnission'medium, for example, line 25 4to .thedisM nominal mid-band of the sending and receiving lters.

In full duplex operation, the DX switch (Figs. 2 and 3) is operated to the FDX position. When the send loop is closed to send a mark signal in full duplex operation, current flows from the -{l3 volt battery at the end of the send loop 2li through resistance Rid and the FDX side of the DX key contacts and then to 130 volt lead. The resulting voltage across resistance RIA is impressed between the grid and cathode of modulator tube 23, Whose cathode is biased to a potential causing it to conduct. The resulting drop in voltage across RI5 depresses the voltage at terminal 2 of coil Tl with respect to terminal 2 of coil L2, causing varistors 22 and 22 to become non-conducting, thereby disconnecting L2 and the associated condenser C2 from the frequency determining circuit of the oscillator. The left-hand portion of the oscillator circuit, being tuned to the marking frequency v1, causes marking current to iiow through oscillator tube 2B. Potentiometer R5 in turn applies a portion of this carrier voltage between the grid and cathode of sending amplier tube 2e. Marking current is thereby impressed on the sending filter 2S and thence on the carrier line 25.

When the operator sends a space, thereby opening the loop 2li and disconnecting the +130 volt battery, the grid of tube 23 drops to 130 volt and since the cathode is then positive with respect to the grid, no current flows through the tube, causing the voltage on terminal 2 of transformer Tl to become positive with respect to terminal 2 of coil L2. The varistors are thereby made conducting so that L2 and its associated condenser C2 are added to the oscillator circuit changing its frequency to the spacing value v2. This frequency is applied to the sending lter 26 and thence to the carrier medium precisely as in the case of a marking signal.

Receiving circuit lig. 3 shows the carrier telegraph receiving circuit. At the receiving end of the circuit the signal is selected by the appropriate receiving lter 3i which accepts a narrow band of frequencies centered about the mark and space frequencies v1 and y2 of the channel to be received. Like the sending filter of Fig. 2, the receive filter 3| provides an impedance transforming structure presenting 600 ohms towards the line but about 140,000 ohms towards the grid-cathode circuit Ri of the rst amplifier stage VEIA.

Tubes V55 (A), V5! (B) and V52 form an amplifier limiter, so that the output of tube V52 remains constant for wide variations in received level. When the received level is low, tube V52 does all of the limiting. The l-megohm resistor 33 in series with its grid prevents the grid from going positive and so confines swings in plate current to the range from to about 10 milliamperes. The output therefore is substantially constant for any signal level at the input of the receiving iilter Si exceeding about -50 decibels per mile. When this level is so high that grid current tends to iiow in tubes Vdi (A) and V5! (B) these tubes also contribute to the limiting action due to the resistance in the grid circuit.

Large carrier amplitudes are limited by tube V5lA and small amplitudes are limited by tu'be V5 iB and/or tube V52.

The resistor R22 constitutes a gain control which is connected between the plate of tube V A and the grid of tube V5IB. It provides the 6 means for adjusting the gain on the limiter amplier aforementioned. The series grid resistor RZI serves to limit the' positive grid swing for large signals and minimizes the amount of selfbias developed across coupling capacitor C6. This arrangement gives substantially symmetrical limiting of the carrier frequency wave form.

The output from the plate of tube V5IB is cou-pled to the grid of` pentode tube V52. The series grid resistor 33 limits the positive grid swing and minimizes the amount of self-bias across coupling capacitor C5 due to grid rectification.

The plate current of V52 passes through the primary side of the discriminator circuit 60. The discriminator consists of two anti-resonant-circuits 62, E3 in series which are tuned respectively to somewhat higher and lower frequencies than v1 and v2, the marking and spacing frequencies respectively. The coils of the two tuned circuits 52, 63 are the primaries of the two independent one-to-one ratio transformers T2, T3, one primary being tuned to parallel resonance at the high frequency edge of the channel band and the other being tuned to parallel resonance at the low frequency edge of the channel band. The secondary windings of T2 and T3 are connected in series. Reversing switch tl provides means for reversing the output connections from the discriminator network.

When the discriminator switch is operated to the HF-iposition, the higher frequency is rectified by rectier 6d and the lower 'by rectifier 65, resulting in direct-current voltages appearing across points XY and YZ respectively. These voltages are poled oppositely and their algebraic sum is applied between the grids and cathodes of tubes V53 and VEA, which are connected in parallel. It follows that when the higher frequency is received, voltage XY predominates and the grids of these tubes are positive with respect to their cathodes, and when ZY is the larger they are negative. Condensers 64 and B5 respectively form a by--pass for the carrier currents.

The function of the limiter is apparent, for since the discriminator would translate either changes in frequency or changes in magnitude appearing at its input into corresponding voltage variations across XY and YZ, it is necessary that magnitude changes be first entirely eliminated by the limiting action of the preceding vacuumtube stages. With the arrangement employed, tube V52 generates a plate-to-cathode alternating-current voltage which is independent of signal magnitude or frequency, but this voltage (or the greater part of it)` is alternately shifted between the input terminals .'i-I and Z-of the discriminator transformer, depending on whether a marking or spacing frequency is received. Fundamentally, the limiting action is secured by producing an amount of amplification in tubes V5i(A) and V5I(B) which is far in excess of that required to obtain the needed voltage across XY and YZ and then severely curbing the magnitude of their outputs. Due to this action, variations in level which affect equally the marking and spacing frequencies cause no signal distortion; in other words, frequency-shift operation gives extremely effective level compensation.

The rectified signals are passed through the low-pass filter Si? consisting of retard coils G1 and condenser 53. The low-pass filter has a cut-oil frequency near 40 cycles and thus also serves to attenuate interference arising from extraneous frequency components differing from assunse.

the carrierV .frequency by more lthan 49 cycles. vA balanced `low-:pass filter structure without mutual inductance is used in order toipresent vhigh and nearly equal impedances to ground. This prevents achangerin the tuning-of the discriminator when reversing switch `fil is operated for reversing connections from thediscriminator. lA positive or negative output may thereby be obtained by the 4marking condition. The discriminator switch el also permits normal operation with a reversal of vrthe markandspace frequency assignments if desired.

The output of the low-pass i'ilter is terminated by resistor Rl! and is applied through lgrid limiting resistor R12 to the grids of output tubes V53 and V55. in order to center the 'demo'du'lated signals on the ygrid characteristic of tubes V53 and V 512 Yand thusavoid biased signals, the mean of the mark and space output voltages from the low-pass filter must be a few volts negative with respect to the cathodes of tubes V53 and V54. This is accomplished by adjustment of the potentiometer te, by which the positive outputcondition may e made 'tob'e of less amplitude than the negative output condition. The potentiometer 6e also provides ineansfior compensating for discrepancies the -discriminator and-deviations in the lmarking and spacing frequencies .from their theoretical values, also for other biases which may originate in the sending terminal.

Tubes V53 and V54 therefore act in Vunison as a single switch whose closing is controlled by positive signals across XZ and whose opening is effected by negative signals across these same points, i. e., they are equivalent to a receiving relay operated by polar signals. When signals are received, this switclr'7 closes 'the receiving loop circuit l5, Fig. 3 if the DX switch is operated to FDX, or loop 243 if the DX switch is operated to HDX permitting Vcurrent from `the +130 volt battery at its distant end to iiow -to the cathodes of tubes V53 and V54 and operate the receiving TTY to marking; when it opens under the Vinluence of a negative voltage this current `is broken and the TTY receives a spacing signal.

The grids of tubes V53 vand V54 are connected through resistor Rl! to the cathodes of `these tubes. Hence, when no carrier is being yreceived from the distant terminal, current will vflow in the .receiving loop holding the Yreceiving teletypewriter closed to marking, since the grids and cathodes oi those tubes are at the same :potential. This is known as the mark-hold feature.

When the higher channel-frequency vis employed for spacing instead oi marking, the righthand part of the oscillating circuit 20 is made inductive instead of capacitive and the discriminator switch t! is operated to LEM- This viinpresses the lower .frequency `through rectier -64 and .the resulting recti'ed voltage appears between points XY.

ln half duplex operation, .the DX switch is operated to the -iDX position. In this condition, the plates of tubes V53 and V are connected to the send loop f2s and receive .loop l5 lis disconnected. When the operator at th-e end of loop 2d is sending signals the `distant station is `normally transmitting a steady marking signal so that the grids of tubes vte .and ves (eig. si are kept continually positive. These `tubes will therefore conduct when the local operator sends a marl; by closing send loop 2li andwillfbe nonconducting when he sends a `space by opening `loop r2d. During the mark, the grid of 'tube '-23 v(Fig. 2) is positive vwith respect to its cathode causing tube -23 to become conducting :and to send out a marking signal over the carrier line a-s described for full duplex operation above. Duri-ng a spacing signal the voltage at the grid of tube 23g, which is connected to -139 volts through resistor Rit falls to i3d volts, tube 23 becomes non-conducting and spacing carrier is transmitted as previously describe-1l.

When mark and space signals are being received from the distant terminal, the local send loop 2G `isnorniall-y Aheld closed to mark. During the receptionof a mark signal, a positive voltage is applied to the grids of tubes V53 and Vet, these tubes cond-uct and current iiows in the loop circuit. This results in a voltage, applied to the grid of tube 23, which is positive with respect to'thelcathode of tube v23 and marking carrier is transmitted over the carrier line as described above. When a spacing carrier signal is Vreceived from thedistant terminal, a negative Vvoltage -is-applied to thegridsof tubes V53 and V554. This reduces the loop current in send loop Ell 'to nearly zero. The potential at the plates of tubes V53 and V54 rises 'to +139 volts in this conditiondue Vto the battery at the end oi the local send loop. Thus 4the voltage applied to the grid of tube 23 becomes even more positive and tube Slay RS to iight the line lamp le.

23 remainsconducting so that an outgoing marking condition .is sustained. Tube therefore cuts @if for a space from thesubscriber loop and conducts for the marking condition and also remains conducting for the space condition toward the loop. rrThis .provides the hali duplex action of the loop circuit.

The resistorRi sh wn inllig.. 2 is eiective in controlling the breair characteristic of the telegraph circuit. `When the subscriber opens the loop to Abreak transmission coming from the receiving terminal, resistor i3 acts to insure that a spacing .potential is applied to grid 'i of modulator tube 23 during intervals when 'the loop circuit is also opened by acut-o condition of tubes Vet and V515.

Supervisory signaling receiving circuit lt is `:further necessary that supervisory relay .RS "be operated whenever the distant subscriber connects carrier lto theA 4line by closing his power switch,'and that it `be released when the carrier is again turned oir. This control is exercised by means oicurrent `from tube 23', Fig. 3. incorn- .ing carrier from the distant station operates re- Aiter the cord is connected to the line jack, release oi' relay RS due to removal oi" carrier, applies ground tothe "jac'kfto ylight the cord supervisory lamp Si. The `signalsl from tube 23' therefore result in the application and vremoval of ground from the ring `of jack -to produce the necessary supervisory signals.

VThe .reception of a supervisory carrier signal through the receiving iilter 3l and its applica- :tion to Vthe :grid of tube V52, results in an altermating-current iiow from the 4plate of tube V52 through blocking condenser lt, resistor 'il and the parallel combination of resistance 'i3 and varstor 'l2 to the negative terminal of the filament battery, thence to gr und and finally back to the cathode of V52. The rectifying action of varistor l2 sets up a direct-current voltage across resistor 'i3 which opposes the 24 volt bias on the grid of tube 23. The low-pass R.-C. lter M, 16, 'il shown in thegrid'circuit of the latter tube removes theycarrier and provides a delay in the building-up of the direct-current grid voltage on this tube. Grid current flow in tube 23' limits the voltage between grid and cathode to zero. When the rectied voltage across vresistor i3 equals or is greater than the -24 volt bias, and the plate of tube 23 is connected through a supervisory relay RS to a +130 volt battery, it furnishes supervisory information to a connecting circuit. When carrier is applied to the receiving filter input at normal level, plate current does not flow for about S) milliseconds, but when the carrier input stops the supervisory relay current falls to zero in about 50 milliseconds. The longer delay insures against false operation of the supervisory relay due to bursts of noise, while the shorter delay provides a fast disconnect signal when the distant subscriber opens his power switch.

Fig. 4 shows a modified frequency shift telegraph carrier oscillator, which differs from the oscillator illustrated in Fig. 2, principally in the form of the varistor network utilized to switch from v1 to v2 and the omission of tuning coil L2 (see Fig. 2).

In the form of varistor network shown in Fig. 4, a bridge Sil of four varistors is utilized and connected to the secondary of transformer Til. Whenever a negative voltage is applied to the varistor bridge Se through switching leads 8i, 82 it eiectively introduces a high impedance or open circuit condition in the secondary circuit of transformer TI i, permitting the oscillator 2G to operate at its natural frequency Whenever a positive voltage is applied to the bridge 80 biasing the varistors in the conducting direction, condenser C2 is thereby connected across the secondary of transformer Ti l, causing the oscillator to operate at a lower carrier frequency v2.

The leads 3|, 32 are associated with a modulator tube such as tube 23 (Fig. 2) which is driven by the signals originating in a connecting directcurrent telegraph circuit illustrated in Fig. 2. The varistor bridge gli thereby functions as a series switch under the control of the aforementioned modulator tube, which conducts for an outgoing mark and is cut off for an outgoing space.

The system described is flexible and particularly adapted for use as trunks between central offices and circuits between central offices and toll subscriber circuits.

It should be understood that any reasonable number of two-way channels may be provided, for example, in the voice band twelve or more two-way channels may be conveniently provided over four-wire circuits and three two-way channels or six or more two-way channels over two-wire circuits in the range between 3850 and 5260 cycles, above the voice band.

It should likewise be understood that the marking and spacing frequencies v1 and 112 may be interchanged with respect to marking and spacing. The necessary circuit modifications therefore would be apparent to one skilled in the art.

What is claimed is:

l. A carrier wave telegraph system comprising a carrier oscillator adapted to generate two discrete frequencies or marking and spacing signals respectively, a frequency determining circuit connected thereto comprising a tuned circuit, an auxiliary reactance, and a bridge of varistors therebetween, and direct-current polar telegraph signals for biasing said varistors from a substantially infinite impedance to a low value to uncouple and couple said auxiliary reactance from said tuned circuit, a supervisory circuit connected to said oscillator and adapted to energize and deenergize it to provide supervisory pulses, a transmission line. connected to said oscillator for transmitting said discrete frequencies and supervisory pulses.

2. A carrier wavetelegraph system comprising an oscillator having a natural frequency determined by a parallel tuned circuit, auxiliary reactances coupled to said tuned circuit, a transformer having varistors in its secondary circuit, polar bias means for varying the impedance of said varistors from a high to a low value to shift the oscillator frequency for marking and spacing, a telegraph circuit for energizing and deenergizing said oscillator to provide supervisory signals, a transmission medium, and means for segregating said supervisory, marking and spacing signals, and means for detecting each signal.

3. .A carrier wave telegraph system comprising a carrier oscillator adapted to Vgenerate two discrete frequencies for marking and spacing signals respectively, a frequency determining circuit connected thereto comprising a tuned circuit, an auxiliary reactance and a network containing varistors coupled to said tuned circuit, and a source of direct-current polar telegraph potentials for biasing said varistors from a substantially infinite impedance to a low value, a supervisory circuit connected to said oscillator and adapted to energize and deenergize it to provide supervisory pulses, and a transmission` line connected to said oscillator for transmitting said discrete frequencies and supervisory pulses.

4. The structure of claim 3, and a tuned receiver for selecting said discrete frequencies connected to said line, an auxiliary receiver branched from said tuned receiver for detecting said supervisory signals, and a telegraph loop connected to said auxiliary receiver.

5. A carrier wave telegraph transmitter comprising an oscillator having a natural frequency determined by a parallel tuned circuit, auxiliary reactance coupled to said tuned circuit, a Wheatstone bridge network of varistors connected to said auxiliary reactance, a polar telegraph circuit connected to said bridge for Varying the impedance of said varistors from a high to a low value to shift the oscillator frequency for marking and spacing, a telegraph circuit for energizing and deenergizing said oscillator to provide supervisory signals, a transmission medium, and means for segregating said supervisory, marking and spacing signals, and means for detecting each signal.

6. A carrier wave telegraph system comprising an oscillator having a resonance frequency determined by a parallel tuned circuit, auxiliary reactance coupled to said tuned circuit for shifting said resonance frequency, varistors connected to said auxiliary reactance for controlling its coupling to said tuned circuit, polar bias means for varying the impedance of said varistors from a high to a low value to shift the oscillator frequency for marking and spacing, a telegraph circuit for energizing and deenergizing said oscillator to provide supervisory signals including a power switch and power source for energizing said oscillator.

'7. A carrier wave telegraph system comprising a carrier oscillator, a plurality of tunable circuits connectable thereto, a variable coupling between;v said tunable -controlvmeansfor said coupling whereby av plucircuits. and vpolar bias rai-itycf- `discrete frequencies is. generated by said oscillator at different times respectively,. a telegraph circuit for energizing and deenergizing saieloscillator toprovide supervisory signals, a line for transmitting said frequencies and supervisory signals, a receiving lter, an amplier limiter, and a` double tuned discriminator circuit connected to said. line for selecting said Vplu-ral frequencies and a separate detector circuit for said supervisory signals branching therefrom.

8. A'carrier Wave telegraph system compris .ing sending. and receiving terminals connected by a transmission line, the sending. terminals comprising an .oscillator adaptedY to. generate two discrete requencies 111 and v2 for marking and spacing. signalsJ respectively, the receiving terminals comprising a receiving lter, amplifier limiters and a discriminator circuit consisting of antiresonant circuits tunedv to. said. frequencies w m. and vz-respectively., varistor diodes connected thereto, a variable potentiometer .connected to said diodes and a pair of vacuum tubes. operating as a single switch connected to. said potentiometer, said potentiometer being adjustable to compensate for signalV bias..

9.. A carrier Wave. telegraph system comprisingA a sending anda receiving terminal connected v nal sending. circuit connected. to said oscillator,

a receiver for said two resonance frequencies in said. receiving terminal and a supervisory signal receiving circuit branching from saidV receiver. 10. The structure yof, claim 9, wherein said resistor is connected to a source, of negativep tential to control. said modulator tube.

LELAND A. GARDNER, 'JOI-IN L. HYSKO.

References Cited in the le of this patent UNITED STATES PATENTS .Number Name Date 1,522,581 Espenschied Jan. 13, l1925 2,474,261 Leibe et al June 2.8,'1949 i FCREIGN PATENTS Nuinber Country Date 698,170- Germany Nov. 4,1940 

